Windows Longhorn build 4015 (Lab06 n) is unleaked. It was shown at WinHEC 2003 to demonstrate the new "Desktop Composition Engine" engine which produces fancy graphical effects without relying on the CPU and slowing down the system. It is a GUI/end-user interface test, as the "lab06" in the string indicates. The same system that was shown at WinHEC 2003 was used in PDC 2003 alongside a system running build 4018. A screenshot of this build is also known to exist, extracted from a Microsoft presentation about the Avalon Graphics Stack (TW04036_WINHEC2004).